Q & A: magnetic work

I know how a magnet works, but what is the power source of a magnetic field? ie: why does a magnet NEVER come of the fridge? PS: Please don't spout some nonsense about the magnet doesn't move so no work is done and no energy is needed. Thanks.
- Jake
England

The magnet doesn't move so no work is done and no energy is needed

Mike W.

(published on 10/22/2007)

If magnets do not do work (because their affect on charged particles is at right angles and Cos 90 is zero so no work is done) then how do magnets make each other move? For example if there is a N side magnet and S side fixed magnet and the N side slides across a table to the S side of the magnet then isn't work done? And since we both know work is done, how is it done? Do the two magnets create an electric field? What about on the microscopic level? What's happening to each of those individual charged particles? And are these principals the same thing that govern the work of a compass? Thank you so much!
- Heather (age 25)
Watkinsville, Ga

This is a really nice question. There are a couple ways in which work is done, i.e. in which the energy stored in the magnetic field is converted to kinetic energy. One is that particles such as electrons are not simply charged particles whose magnetic fields come only from their motion. They also have intrinsic spin magnetism even when they are at rest.  So there's a term in the potential energy which depends on the product of this intrinsic magnetism and the magnetic field. If the electrons move the right way in the field, that potential energy drops and work is done on the electrons.
 Even in the case of magnetism due to currents flowing, not spin, work can be done.
For a fuller discussion, see http://van.physics.illinois.edu/qa/listing.php?id=17176


Mike W.

(published on 08/15/08)

I found you answer to the magnetic work question most unsatisfactory. Assuming that the principal behind "the magnet doesn’t move so no work is done and no energy is needed" is true for all situations means that to hold an apple in the air completely still requires no energy to be used in the muscles. Since everybody knows that holding an apple up requires energy how do you explain the "the magnet doesn’t move so no work is done and no energy is needed" argument?
- Muhammed (age 17)
Sir George Monoux College, London

Now that's a nice question.

If you hold an apple up in certain ways, it feels like you're doing work but in other ways it doesn't. If you lie on the ground with an apple on your belly (or hand) it doesn't feel like you're doing work. If you hold the apple out in your hand, it does feel like you're doing work. There must be some difference.

The feeling of doing work is a way of registering what's going on in your muscles. If they're using up chemical fuel to make one muscle protein (actin) move with respect to another (myosin) then you sense that you're doing work. Yet if the apple is just staying at a fixed height, no work is being done on it, in the physics sense. Why do you need to use energy in your muscles just to keep the apple up?

If you're using your muscles to hold up the apple, it would slip a little because the actin and the myosin don't stick perfectly. You have to use up some energy to make up for that slippage.

Here's an analogy. Say you have a car parked on a slope, with very high friction between the tires and the road. The car doesn't need to do any work to stay up. You can leave the engine off. Now say that the tires slipped a little on the road. The car will fall unless you turn on the engine and get the wheels turning, expending fuel. Keeping the car in place will feel a lot like accelerating it, since the motor will vibrate, fuel will be used up, exhaust products made, etc.

Now back to the magnet. There aren't any little molecules slipping when the magnet sticks well. It's like the car with high-friction tires. So no energy has to be expended.

Mike W.

(published on 12/06/11)

I have a different question related to gravity and magentism. I bought some neodymium magnets for work (about 3/16" dia. x 3/32") for a project. After finishing, I put them on my door to hold papers up. I then used one to hang a steel rod from the cieling tile channel (about a 2 foot x 3/32" rod). It's been hanging there for months. Energy has to be expended counteracting the gravitational energy pulling it down to the ground. Not only is the magnet holding the weight of the rod, but also the weight of the magnet and holding both up against the steel ceiling grid. The energy needed to hold this rod suspended above the ground seems infinite at this point. Since the energy is much greater than the gravitational pull, there should be some way to capture this energy and convert it to another form of energy such as electricity. I'm sure you see where this is going, a perpetual motion machine that would generate energy. I remember seeing a wheel using these magnets on the internet that once it was started, continued to run. I am sure there are bearing friction losses and wind resistance that it has to overcome. If you reduced these by placing it in a vacuum and using magnetic levitation frictionless bearings, you could concevably generate excess energy? Can you tell me where this idea fails?
- Tim McCreary (age 46)
Roaring Spring, PA USA

Tim- I've switched the thread of your follow-up to one that provides a better introduction.  Your argument goes wrong right at the start- the magnet is expending no work.  As we argued before, it's just like a car parked on a slope. If the wheels aren't slipping, the engine doesn't have to do any work.

Mike W.

(published on 04/18/13)